A novel band-rejection filter based on a Bragg fiber with a defect layer is proposed. A defect layer is introduced in the periodic high/low index layers in the cladding of the Bragg fiber, which results in large confinement loss for some resonant wavelengths inside the band gap range of the Bragg fiber. A segment of the Bragg fiber with a defect layer can be used as a band-rejection filter, whose characteristics are mainly determined by the structure of the Bragg fiber. The simulation results show that the bandwidth of such a band-rejection filter is dependent on the number of the periodic high/low index layers in both sides of the defect layer in the cladding of the Bragg fiber.